He'll have to prove himself の意味は？

彼は良い男性だと私は思うのだが、本当にそうなのかどうかは、彼が自分で証明してみせないといけない。

彼はやっぱり良い男性だと私たちが納得するようなことを、彼はする必要がある。

日本語で簡単に言うと、「どう良いのか見せてもらいたい」でしょうか。

This is not a piece of advice to you I'm talking to myself for my practice of writing skills. on the site, I suppose it would be a common knowledge that on this screen right crick of the mouse would lead Google Translation start to work. if you have clicked "日本語に翻訳(T)" you could read the pages in Japanese language. You'd be able to comprehend your question.
:8 Ways He Should Prove Himself Before You.
https://herway.net/he-should-prove-himself/
:GT.
https://translate.google.co.jp/?hl=ja&sl=en&tl=ja&text=He%27ll%20have%20to%20prove%20himself&op=translate;
:prove yourself (to be) something.
https://www.ldoceonline.com/dictionary/prove-yourself-to-be-something;
:merriam-webster.
https://www.merriam-webster.com/dictionary/prove%20oneself